Heat-sensitive copy-paper



March 21, 1961 c. A. KUHRMEYER 2,976,415

HEAT-SENSITIVE COPY-PAPER Filed July 21, 1958 obsarbenf flea? sensifive /y. 2 cop va aerabsogofive reac7l've /ayer /Z supp or fin lay er j/VVENTOR CA 12L A. KUHRMEYER ATTORNEYS United States HEAT-SENSITIVE COPY-PAPER Filed July 21, 1958, Ser. No. 749,708

4 Claims. (31. 250-65) This application relates to heat-sensitive copy-paper having a chemically reactive visibly heat-sensitive coating, and in particular to copy-papers capable of copying at reduced radiation intensity, or having chemically reactive components requiring normally excessively elevated temperature conditions for reaction, or both. The invention also relates to apparatus for use in conjunction therewith.

Many chemically reactive visibly heat-sensitive copypapers are known which are capable of providing excellent reproductions of printed originals by front-printing thermographic copying procedures involving intense brief irradiation of a graphic original through the radiationtransmissive copy-paper. Radiation is normally obtained from a heated tungsten filament, the radiation being absorbed in the printed image areas of the graphic original and the resulting heat pattern being then transmitted by conduction to the overlying copy-paper to produce the desired visible change resulting in a reproduction of the printed original. Although high intensity radiation is thus obtained, the intensity is nonetheless limited both by the nature of the tungsten filament and because of the possibility of degradation of the original if excessively overheated. The present invention provides means for operataing well within the limitations thus prescribed, as well as at less than normal intensity and temperature. The invention also provides for the utilization of chemically reactive systems 'which would normally be inapplicable to therrnographic printing processes since the temperatures required to promote the visible reaction are higher than those normally available in these processes as ordinarily employed.

The copy-papers of this invention are also useful for atent 2,976,415 Patented Mar. 21, 1961' cal except for the further addition of 0.66 part, i.e. ap-

reproducing graphic originals by means of radiation obtained from gas flash tubes. As compared with prior art heat-sensitive copy-papers, the products of this invention are found to produce useful copies at much reduced intensity of irradiation from such flash lamp sources, measured in terms of reduced voltage or reduced capacitance in the power employed therewith.

In the drawing, Figure 1 schematically indicates simplified apparatus for use with the copy-paper of this invention and employing a gas flash source of radiation, and Figure 2 represents in cross-section a typical heatsensitive copy-paper made in accordance with the invention.

The copy-paper 10 of Figure 2 comprises a supporting base or backing layer 12 carrying a coating 11 containing the inter-reactive chemical components together with a radiation-adsorptive colored pigment, all as will be hereinafter further described in terms of illustrative but non-limitative specific formulations and examples.

In one series of copy-papers made in accordance with this invention, a mixture of 10 parts by weight of ethyl cellulose binder, 10 parts of basic lead carbonate, 2 parts of thiourea and 156 parts of alcohol is prepared by ball milling. A second mixture is similarly prepared, identiproximately 3% based on the total non-volatile components, of carbon black. Mixtures of the two are also prepared, at intermediate proportions of carbonblack. The several compositions are coated on 25 lb. map overlay tracing paper, dried, and tested as heat-sensitive copypaper by front-printing technique, using a tungsten filament source of radiation at maximum safe operating conditions. In the absence of carbon black, no visible print is obtained. At 1.0, 1.5 and 3.0% of carbon, a reproduction of the graphic original is formed.

With the larger proportions of the absorptive pigment, the background color of the resulting copy becomes increasingly intense, reducing the contrast. Introduction of titanium dioxide into the coating reduces the background color but the image areas are then rendered less intense, again resulting in low contrast. It is therefore desirable to maintain the proportion of carbon at as low a value as possible while still providing for the adsorption of suflicient radiant energy to permit the sheet to undergo reaction at the image areas.

In another series of copy-papers, the chemical reactants are separately compounded and coated on the paper support, and the paper is exposed to radiation from a gas flash source through a stencil having a perforated or cut-out image portion in an opaque backing.

The first coating contains 19.24 parts by weight of ferric stearate, 4.63 parts of ethyl cellulose, and varying amounts of carbon black, the whole being dispersed in 75 parts of acetone. The second coating contains 10 parts by weight of gallic acid, 20 parts of ethyl cellulose, and similar proportions of carbon black, dispersed in 180 parts of acetone. The compositions are prepared by ball milling until smooth, and are coated at a wet thickness of about 2 mils for the ferric stearate mixture and 1 for the gallic acid mixture.

The proportion of carbon is increased stepwise from 0 to 6% based on the total non-volatile components.

The flash lamp is supplied with power from a 262 mfd. capacitor charged to various voltages. At 420 volts an indistinct image is formed in the sheet containing 0.6% carbon, and a distinct image in the sheet containing 1.5% carbon; at lower proportions no image is formed. At 440 volts a distinct image is formed at 0.6% carbon, a decreasingly distinct image at 0.3, 0.1 and 0.05% carbon, and no image at 0% carbon. The carbon-free sheet forms no visible image at voltages less than 500, and requires at least about 520-540 volts to produce an image equivalent to that produced at 440 volts in the sheet containing 0.6% carbon. sults are obtained by maintaining a constant voltage and by varying the capacity. In either case, it will be apparent that reduction in power input is made possible by the principles of this invention, thereby eflect-ing economy in equipment, increase in safety, and other advantages.

Although the specimens prepared with 6% or somewhat less carbon provide readily distinguishable copy, they are high in background color and therefore undesirably low in contrast. Some background color is noted at 1.5 and even at 0.6% of carbon. While the background color can be reduced by inclusion of small amounts of white pigments such as titanium dioxide in the heat-sensitive layer, the color intensity of the image is thereby reduced, again reducing the contrast. However at lower amounts of carbon the background color is not visibly distinguishable from that of the copy-paper prepared without the addition of carbon.

Coating the reactive materials in two layers is desirable to ensure physical separation and prevent premature reaction of the chemical reactants, but is not essen Generally the same retial'. Many heat-sensitive compositions containing iron soaps and phenolic reactants are effectively applied simultaneously in a single layer. The inert absorptive pigment is added to the composition in the same total proportion and in the same manner as hereinabove described.

In another similar series of copy-papers in which the chemical reactants involved consist of a ferric soap and a phenolic component, a white pigment, in this case zinc oxide, is employed in place of the carbon black and in much increased amounts. Under the same conditions of irradiation, i.e. using the gas flash lamp system, the unpigmented specimens require a voltage of 620, whereas the specimens prepared with from 10 to 50 percent of zinc oxide are found to operate successfully at 5 60 volts. Titanium dioxide is similarly effective in increasing the absorption of radiation from the gas flash source.

In the front-printing technique hereinbefore described the radiant energy is partly absorbed inthe copy-paper to a degree sufiicient to increase the temperature of the heat-sensitive layer to a point just short of reaction. The additional heating effect obtained on absorption of further portions of the radiation in the printed image areas of the original is then sufficient to cause the desired chemical reaction to proceed at corresponding areas of the copy-paper. For this technique, apparatus is required which will hold the copy-paper and original in mutually heat-conductive contact and protected from external heat-loss.

Figure 1 illustrates another way in which the heatsensitive copy-papers of this invention may be employed. In this method the copy-paper is placed over the graphic original 14, with the heat-sensitive coating 11 in contact with a transparent glass panel 16 and separated from the printed surface of the original, of which a copy is desired, by the supporting layer 12. The original rests on a supporting base 15. Radiant energy is provided by gas flash lamp 17 operated from power source 18 in known manner. The lamp may conveniently be a Pyrex glass or quartz tube about ten inches long, having an electrode at each end, filled with xenon or other inert gas under reduced pressure, and externally wound with an open Wire spiral. The spiral and electrodes are suitably connected to the appropriate igniting and power source through a control switch, indicated schematically in the drawing. In this copying procedure the radiant energy is first partially absorbed in the coating 11, the temperature of which is raised to a point just below that required to produce a visible change. The remaining radiation passes through the supporting layer 12 to the surface of the graphic original. At this surface, radiation is absorbed in the printed image areas 13 and reflected from the intervening areas. The reflected rays re-enter the corresponding portions of-the sensitive coating 11 and are partially absorbed, causing further increase in temperature to a degree sufiicient to cause immediate darkening of the heat-sensitive layer. The heating effect 4" occurs within the sensitive layer itself, and, because of the nature of the flash discharge, reaction occurs before the heat pattern is dissipatedby conduction. Similarly, any heat produced at the printed areas 13 is ineffective in causing a color change in the heat-sensitive layer.

What I claim is as follows:

1. A heat-sensitive copy-paper adapted for making reproductions of graphic originals by front-printing with radiant energy as herein described and comprising a supporting web and a chemically reactive, visibly heatsensitive coating containing a radiation-absorptive pigment in an amount suflicient to substantially increasc the radiation absorptivity of said copy-paper without significantly reducing the contrast of the resulting reproduction.

2. Apparatus for making reproductions of graphic originals by front-printing with radiant energy as herein described and comprising a gas flash lamp member, power supply means for flashing said lamp, means for supporting a graphic original with its printed surface, of which a reproduction is desired, in position for irradiation, and means for holding a partially radiation-absorptive, chemically reactive, visibly heat-sensitive copy-paper closely adjacent said printed surface and in position for intercepting the radiation from said lamp member to said original.

3. A heat-sensitive copy-paper adapted for making reproductions of graphic originals by thermographic frontprinting procedures, comprising a supporting Web and a chemically reactive, visibly heat-sensitive coating containing a significant small proportion of a radiation-absorptive dark-colored pigment sulficient to substantially increase the radiation absorptivity of said copy-paper without significantly reducing the contrast of the resulting reproduction, said copy-paper undergoing a visible change under thermographic front-printing procedures at a radiation intensity insufficient to cause said visible change to occur in an otherwise identical copy-paper free of said pigment.

4. A heat-sensitive copy-paper adapted for making reproductions of graphic originals by thermographic frontprinting procedures, comprising a supporting web and a chemically reactive, visibly heat-sensitive coating containing a minor proportion of a radiation-adsorptivc white pigment sufiicient to substantially increase the radiation absorptivity of said copy-paper, said copy-paper undergoing a visible change under thermographic frontprinting procedures at a radiation intensity insufficicnt to cause said visible change to occur in an otherwise identical copy-paper free of said pigment.

References Cited in the file of this patent UNITED STATES PATENTS 2,663,654 Miller et a1. Dec. 22, 1953 2,740,895 Miller Apr. 3, 1956 2,844,733 Miller et a1. July 22, 1958 2,880,110 Miller Mar. 31, 1959 

